1. Field of the Invention
The present invention relates to cameras used to image human-readable and machine-readable markings, and more particularly, to an improved camera apparatus for capturing and decoding images of one and two dimensional bar codes, matrix symbols and optical character recognition markings from various surfaces, including rough or reflective surfaces.
2. Description of the Related Art
Many industries, including those involving component assembly, processing, or inventorying, use an identification system in which products are marked with machine-readable parts identification markings, such as bar code symbols, two-dimensional matrix symbols, and optical character recognition symbols, all of which contain information about the products.
A number of different readers and laser scanning devices have been developed to decode symbol patterns to a multiple numeric or alpha/numeric representation. These "optical scanners" are available in numerous configurations, and are either incorporated into fixed station apparatus or portable apparatus.
Portability of an optical scanner enhances its ability to inventory products on shelves and to track portable items such as files, spare parts or small equipment. Portable scanning devices typically allow the user to scan the coded symbols at various distances from the surface on which the coded symbol is located. One problem associated with such scanners is that they are unable to read coded symbols located on highly reflective or polished surfaces. A second problem is their high cost of manufacture.
Coded-symbol scanners that use light emitting diodes (LEDs) as a light source and a charged coupled device (CCD) or complementary metal-oxide semi-conductor (CMOS) as detectors are also known. These image camera scanners are generally referred to as "CCD" and "CMOS" scanners. While CCD and CMOS scanners have the advantage of being less expensive to manufacture, they nevertheless require that the equipment be maintained in contact, or at least in close proximity, with the surface on which the symbol is located.
However, the imaging capabilities of such cameras are typically adversely affected by specular reflection from smooth, polished, or other highly reflective surfaces, or by shadowing produced by minor surface defects, or by patterns or surface textures produced as a result of machining.
The solid state image sensor used in CCD and CMOS cameras comprises a matrix array of light sensitive monolithic silicon chips ("pixels") which absorb photons. A typical 512.times.512 matrix sensor array contains 262,144 chips.
During operation, the solid-state image sensor converts incident light to electric charge which is integrated and stored until the time of readout. The integrated charge is directly proportional to the intensity of the light on the sensing elements. Readout is initiated by a periodic start or transfer pulse. The charge information is then sequentially read out at a rate determined by clock pulses applied to the image sensor. The output is a discrete time analog representation of the spatial distribution of light intensity across the array.
One notable disadvantage of using CCD cameras in imaging ("capturing") machine-readable symbols is a phenomenon known as "blooming", a condition which can severely degrade the performance of the image sensor and can cause problems with the camera circuitry. Blooming occurs when an excessive number of light generating electrons are produced. The result is that bright parts of the image smear and spread out into the surrounding darker areas of the image creating false responses from the pixels in those areas. The effect appears similar to the petals of a flower blooming out from a bud; hence, the name "blooming".
The total charge in any given pixel is the result of photon absorption which creates electrons that accumulate over the integration period; thus, the total photo-electron generation ("photocurrent"). The brighter the light intensity, the higher the photocurrent. Excessive charge can be created by either excessive illumination or too lengthy an integration time. The excessive charge cannot be completely discharged within the average time a human can hold such cameras still (normally 1/60 second), so it leaks past the transfer gate of the CCD and appears as a vertical stripe on a video monitor. An even greater intensity light produces a charge that not only leaks past the transfer gate, but also past the channel stoppers of the other pictures, thus causing a washout area in the video image. This condition is technically known as depth overflow.
These problems generally arise as a result of the use of direct lighting, and can be overcome in most fixed station camera applications by controlling the external lighting conditions using special equipment, as for example, portable studio lights, filters and reflectors. This external equipment, however, is not practical for use in the field where lighting conditions change on a continual basis.
For example, to overcome the problem of specular reflection, field use cameras can be operated in a "touch" mode, where the optical windows of the cameras are pressed against a substrate's surface atop the coded symbol to be imaged in order to block out all incident light. The surface is then illuminated with a built-in light source which emits light having an optimum intensity, angle and wavelength. Cameras of this type are extremely effective for imaging high quality paper labels applied to flat surfaces, but they are ineffective for acquiring images from rough or reflective materials or curved surfaces.
Moreover, various part identification and data capture companies have tried to create non-contact (i.e., "non-touch") cameras which are able to capture and decode machine-readable symbols from non-paper substrates, but these efforts have not produced much success.
Exemplary symbol reading cameras are known from U.S. Pat. No. 4,742,220 to Beyor, U.S. Pat. No. 3,961,198 to August, U.S. Pat. No. 4,743,773 to Katana et al., U.S. Pat. No. 4,818,847 to Hara et al., U.S. Pat. No. 4,825,057 to Swartz, U.S. Pat. No. 4,900,907 to Matusima et al., U.S. Pat. No. 4,908,500 to Bamberger, U.S. Pat. No. 4,983,817 to Dolash, U.S. Pat. Nos. 5,291,009 and 5,354,977 to Roustaei, and U.S. Pat. No. 5,350,909 to Powell et al.
None of these cameras, however, are effective in any mode to permit optical imaging of encoded symbols from polished or reflective surfaces.